The present invention relates to an image display apparatus such as a head-mounted display (HMD) capable of presenting a wide viewing angle image by using plural display elements.
It is necessary for an image display apparatus such as an HMD, which enlarges an original image displayed on a display element to present a display image to eyes of a user, to have a wide display viewing angle and a high display resolution in order to increase realistic sensation. On the other hand, it is necessary for such an image display apparatus to be compact and lightweight so as to reduce a sense of discomfort and a sense of fatigue even when it is mounted on the user's head. However, a wider viewing angle increases the size of an optical system of the apparatus. Further, an appropriate-sized display element having a large number of pixels and being capable of employing an enlarging magnification reasonable in optical design is extremely rare.
In contrast thereto, an image display apparatus has been conventionally proposed in which plural small-sized display elements each having a small number of pixels respectively display partial images corresponding to plural viewing angle areas different from one another of a single image to increase the number of pixels for displaying the entire single image and realize a wide display viewing angle.
For example, Japanese Patent No. 3363647 discloses an image display apparatus in which plural mirrors provided for plural display elements respectively reflect image light fluxes from the plural display elements to synthesize them to present a single image formed by connecting plural partial images corresponding to plural viewing angles through a magnifying optical system.
Further, Japanese Patent No. 3524569 discloses an image display apparatus having two display elements and two returning mirrors symmetrically disposed respectively with respect to viewing axis, which presents a single image formed by connecting two partial images.
Moreover, Japanese Patent Laid-Open No. 2001-042250 discloses an image display apparatus in which plural coaxial optical systems with narrow viewing angles are simply arranged such that an optical axis of each coaxial optical system passes through an eye (pupil) of a user.
Further, image display apparatuses which use a prism type optical element having plural optical surfaces to fold an optical path in the optical element and thereby a compact optical system is achieved. For example, Japanese Patent Laid-Open Nos. 11-326820 and 10-246865 disclose an image display apparatus using an optical element having five or three optical surfaces.
The image display apparatus disclosed in Japanese Patent Laid-Open No. 11-326820 uses an optical element on which five optical surfaces are formed by joining two prisms each having three surfaces and being disposed symmetrically with respect to a viewing axis. The optical element internally reflects image light fluxes each corresponding to a half-viewing angle and respectively proceeding from two display elements in regions of the two prisms to guide them to an eye.
Further, the image display apparatus disclosed in Japanese Patent Laid-Open No. 10-246865 guides image light fluxes each corresponding to a half-viewing angle and respectively proceeding from two display elements to an eye via three transmissive and reflective surfaces on the optical element.
The image display apparatuses disclosed in Japanese Patent Nos. 3363647 and 3524569 and Japanese Patent Laid-Open No. 2001-042250 have tendency of increasing the size of the entire optical system because an optical path is folded outside a magnifying optical system or plural optical systems similar to each other are arranged.
Further, the image display apparatuses disclosed in Japanese Patent Laid-Open Nos. 11-326820 and 10-246865 may be attempted to reduce the size of the optical system by increasing space efficiency. However, in the image display apparatus disclosed in Japanese Patent Laid-Open No. 11-326820, a thickness of its optical element is reduced in a viewing axis direction, but on the other hand, it is difficult to reduce the size of the optical element in its width direction orthogonal to the viewing axis direction.
Moreover, in the image display apparatuses disclosed in Japanese Patent Laid-Open Nos. 11-326820 and 10-246865, light emerging from the display element in a direction different from a direction toward an exit pupil which is an original viewing angle direction may be transmitted through an internal reflective surface without fulfilling a total reflection condition to directly reach the eye. Such light is recognized by the user as unnecessary light, which deteriorates a quality of a presented image.